Field of the Invention
The present invention relates to a heating device for heating an object, a fixing device including the heating device and fixing a toner image onto a sheet, and an image forming apparatus including the heating device, such as a copier, a printer, or a fax machine.
Description of the Related Art
In a fixing device serving as a heating device for nipping, conveying, heating, and pressurizing a sheet having an unfixed toner image formed thereon, thereby fixing the toner image onto the sheet, the heat capacity of a heating member is minimized so as to enhance energy efficiency. Moreover, there has been proposed a fixing device capable of reducing the energy (power) required for increasing the temperature of a heating member to a fixing operable temperature.
The fixing device includes, for example, a heating body such as a heater which is fixed and supported, a heat-resistant fixing film serving as a band-shaped rotation unit which rotates while being press-contacted with the heating body, and a pressure member such as a roller for bringing a sheet as a material to be heated into close contact with the heating body through the intermediation of the fixing film. There has been proposed a fixing film heating system of heating and fixing an unfixed toner image formed on the surface of a sheet onto the surface of the sheet by imparting heat from the heating body to the sheet via the fixing film.
The fixing device of the fixing film heating system can use a heating body that rapidly increases in temperature, and a thin film having a low heat capacity. Therefore, the fixing device has advantages in that power can be saved and the printing standby time period can be shortened (quick start can be realized), with the result that an increase in temperature in an image forming apparatus can be suppressed.
However, although power can be saved and the printing standby time period can be shortened (quick start can be realized) by adopting a thin film as the fixing film, wrinkles are formed on a sheet when the sheet is fixed and conveyed in some cases.
When the fixing is performed to small-sized sheets consecutively, a temperature difference is caused between a part through which the small-sized sheets pass and a part through which the small-sized sheets do not pass in a rotation axis direction of the pressure roller which is being driven. The temperature difference causes a rotation speed difference in the rotation axis direction of the fixing film which is rotated in association therewith, with the result that the fixing film is warped. The warp causes a change in the entry timing of a sheet leading edge in a longitudinal direction (rotation axis direction) of the fixing film with respect to a fixing nip part (abutment part between the fixing film and the pressure roller), resulting in wrinkles on the sheet.
Regarding countermeasures against the occurrence of wrinkles on a sheet, Japanese Patent Application Laid-Open No. H11-2977 discloses a technology of preventing warp of the fixing film by setting a gap between the fixing film and a support member in the fixing film (difference between an inner circumferential length of the fixing film and an outer circumferential length of the support unit) to be small.
However, the countermeasures against the occurrence of wrinkles disclosed by Japanese Patent Application Laid-Open No. H11-2977 are insufficient, and in some cases, wrinkles may be formed on a sheet or a phenomenon may occur in which the fixing film is biased to the center thereof and is overlapped at the center (hereinafter referred to as “film overlapping”).
In Japanese Patent Application Laid-Open No. H11-2977, although a gap between the fixing film and the support unit supporting the fixing film (difference between the inner circumferential length of the fixing film and the outer circumferential length of the support member) is set to be small, a special setting is not performed with respect to a gap between the fixing film and the support unit on an inner side of the fixing film in the longitudinal direction (rotation axis direction) of the fixing film.
In Japanese Patent Application Laid-Open No. H11-2977, as in Comparative Example 1 illustrated in FIG. 9, a protrusion 118g is provided only at a position opposed to a central part in a longitudinal direction (right and left direction of FIG. 9) of a fixing film 118b on an upper surface 118r of a heater stay 118f for supporting a heater 118a serving as a heating body.
In the configuration of FIG. 9, a gap between the fixing film 118b and the support unit is small in the central part (protrusion 118g), compared to that in end parts. This configuration causes film overlapping that is a phenomenon in which the fixing film 118b itself is biased to the center in the longitudinal direction when a sheet, to which the fixing is performed, passes or causes wrinkles which are a sign of film overlapping to occur on a sheet. Further, the configuration strengthens the force that causes the fixing film 18b to be biased to the center thereof in the longitudinal direction, resulting in that film overlapping and wrinkles on a sheet cannot be sufficiently prevented.